Sprinkle the breasts with salt an pepper. Combine the soy sauce, maple syrup, oil, vinegar, garlic and pepper flakes in a dish large enough to hold the breasts in 1 layer. Blend well. Add the chicken breast halves and coat them with the marinade. Arrange them skin side up in the dish and cover them with plastic wrap. Let stand 15 minutes, or until ready to cook. Meanwhile, preheat broiler or a charcoal grill. Arrange the breasts skin side down in 1 layer in a baking dish. Place the chicken under the broiler about 6: from the heat. Leave the door partly open. Cook about 5 minutes and turn the pieces. Cook skin side up for about 2 minutes. Shift the pan to the lower rack so that the pieces are about 12" from the heat. Continue broiling skin side up with the broiler door partly open about 5 minutes longer. Turn the pieces skin side down and broil 5 minutes longer on the bottom rack. Turn the pieces skin side up and pour the remaining marinade over all. Place the chicken about 6" from the heat and broil about 2 minutes. (If you barbecue, cook the chicken on the hot grill, covered. The cooking time is the same.)